To lighten the weight of automobiles, which may have a beneficial effect on global environmental problems, it can be desirable to make the steel used in automobiles as high in strength as possible. However, steel sheet having a high strength may often exhibit a reduced elongation or r values and lower formability. One approach to solve this problem relates to a technique for hot shaping steel and utilizing the heat to raise the strength, which is described, e.g., in Japanese Patent Publication (A) No. 2000-234153. This technique includes suitably controlling the steel composition, heating the steel in the ferrite temperature region, and utilizing precipitation hardening in that temperature region to increase strength.
Further, it has be proposed to provide a high strength steel sheet having a yield strength that is greatly reduced at a shaping temperature to a value much lower than the yield strength at ordinary temperature, which may improve precision of press-forming as described, e.g., in Japanese Patent Publication (A) No. 2000-87183. However, such techniques may be limited with respect to the strength that can be obtained. Alternatively, a high strength may be obtained by heating steel to a high-temperature single-phase austenite region after shaping and, in a subsequent cooling process, transforming the steel to a hard phase as described, e.g., in Japanese Patent Publication (A) No. 2000-38640.
However, heating and rapid cooling after shaping may lead to problems in obtaining shape precision. Techniques which may be used to address this issue by heating steel sheet to a single-phase austenite region and cooling the steel in the subsequent press-forming process are described, e.g., in SAE, 2001-01-0078 and in Japanese Patent Publication (A) No. 2001-181833.
When processing high-strength steel sheet which may be used, for example, automobiles etc., formability (or shapeability) can be more significantly reduced at higher strengths. For example, a member having a high strength, e.g., of over 1000 MPa, may exhibit undesirable hydrogen embrittlement (which may also be referred to as season cracking or delayed fracture). When such materials are used as hot-press steel sheet, there may be little residual stress due to the high temperature pressing, but hydrogen may enters the steel at the time of heating before pressing. Further, residual stress associated with subsequent working can lead to greater susceptibility to hydrogen embrittlement. Therefore, merely pressing at a high temperature may not solve such problems. It may be desirable to optimize process conditions for the heating process and for subsequent integrated processes.
To reduce residual stress in shearing and other post-processing operations, it may be sufficient to provide a reduced strength of the parts to be post-processed. Techniques for lowering the cooling rate of material regions to be post-processed, so as to reduce hardening and thereby lower strength in these regions, are described, e.g., in Japanese Patent Publication (A) No. 2003-328031. When using such techniques, the strength of certain portions of a workpiece may be lowered, which can in turn allow for easier shearing or other post-processing mechanisms. However, the mold structure may become complicated—which can be economically disadvantageous. Further, hydrogen embrittlement is not alluded to at all in this reference. Thus, even if the steel sheet strength can be reduced somewhat and the residual stress after post-processing may also be reduced to a certain extent, hydrogen embrittlement may still occur if hydrogen remains in the steel.
Thus, there may be a need for improved high-strength materials and methods for providing them which overcome the above-mentioned deficiencies.